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Manhattan transfer

Transport - Technical challenges abound on the complex £395M Fulton Street interchange upgrade.

To the casual observer, Fulton Street in downtown Manhattan is little more than a run down shopping area whose main point of interest is the Ground Zero site at its western end. Here and there narrow Subway entrances are squeezed between shop fronts, but even the largest entrance is far from conspicuous.

Yet below Fulton Street is one of the Subway's busiest interchange stations. Around 300,000 passengers a day pass through the station's pokey and confusingly signed underground corridors, switching between the five lines passing through it or making their way to and from trains and station entrances.

This year New York's Transit Authority, advised by consultant Arup, is to start a major $750M (£395M) overhaul of the station, making it more accessible to commuters and easier to find for tourists.

The centrepiece of this will be a spectacular glass and steel 'occulus' building designed by the UK's Grimshaw Architects, to serve as the new main entrance. The structure comprises a lattice framed, truncated cone which will rise eight storeys above ground and funnel natural light into the underground concourses.

It will provide a visual focus for a growing influx of tourists expected to visit the area after redevelopment of the World Trade Center site, following its destruction in the 9/11 attacks.

But it is the work below ground which is likely to tax engineers' brains the most.

Under Broadway, which cuts across Fulton Street, a new underpass has to be excavated to increase underground capacity and ease passenger flows from the main entrance.

Most of this will be done in an open cut, but one section has to be dug below the two platforms serving Lines Four and Five, which share track at this point.

For this, contractors will need to insert a cat's cradle of piles and support beams to carry the live Subway while excavation work takes place below.

'We will need a fairly dense arrangement of underpinning piles through the platforms and trackbed slabs, ' says Arup structural engineer Tom Rice.

Under Fulton Street itself, a mezzanine passageway sandwiched beneath the roadway and above the two platforms serving the A and E Lines will also present some serious challenges.

The mezzanine level is the base of a cut and cover box which currently rests in the ground, independent of bored tunnels housing the A and C Lines. Vertical loads are transferred to foundations via closely spaced steel columns that pass through the platform and on to a base slab between the running tunnels.

'There is no scope for inserting new foundations because of the running tunnels, ' says Rice. 'So we have created a new structure with a floor beam above the existing floor. We have cantilevered it out above the tunnels but it is supported on the platform columns.' Building the new structure will involve opening up the 10m deep, 8m wide mezzanine box to street level, and creating a new one just over 11m wide - the same width as Fulton Street itself.

This will entail inserting pretensioned temporary ground anchors to prevent heave in the tunnels and platforms during construction, as demolition of the existing box reduces load in the surrounding water bearing glacial sands and silt.

Two sewers running along the outer walls of the mezzanine box also need relocating, as will other services just below street level.

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